Printer

ABSTRACT

A second driven roller that is rotated by following a second driving roller has a larger outer diameter than a first driven roller. A front edge of a recording sheet being lifted up from a first nip contacts the second driven roller, and hence the front edge does not directly hit the first driven roller. The second driven roller rotates at a higher rotational speed than a rotational speed of the first driven roller. The second driven roller gently flips the front edge of the recording sheet which has contacted the second driven roller, and leads the front edge quickly to the first nip. The front edge of the recording sheet is smoothly led to the nip.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a printer.

2. Description of the Related Art

A recording sheet has to be conveyed at a constant speed during printing in either of an inkjet printer or a thermal printer that conveys a recording sheet to the position of a print head and performs printing.

A recording sheet is typically conveyed by rotating a plurality of pairs of conveying rollers provided in a recording area.

Each pair of conveying rollers includes a driving roller that is connected to a motor and rotates by itself, and a driven roller that is pressed to the driving roller and rotates by following the driving roller. When a recording sheet is pinched by the driving roller and the driven roller, the recording sheet can be conveyed. The rotational speed of the pair of conveying rollers is controlled, so that the recording sheet is conveyed with high precision.

However, when the front edge of the sheet enters a nip of the pair of conveying rollers, the conveying speed of the recording sheet may be temporarily decreased, and hence, the quality of an image printed on the recording sheet may be degraded.

To prevent the conveying speed of the recording sheet from being decreased, the following techniques have been suggested.

In the technique described in Japanese Unexamined Patent Application Publication No. 2006-248670, the linear speed of a registration roller is increased by rotation controlling means immediately before the front edge of a sheet enters a fixing section.

In the technique described in Japanese Unexamined Patent Application Publication No. 7-271126, a conveyance guide plate is provided. The conveyance guide plate is curved to cause a sheet to form a smooth loop and arranged between upstream and downstream pairs of conveying rollers. In addition, a pair of auxiliary conveying rollers is provided in the middle of the conveyance guide plate. The pair of auxiliary conveying rollers moves up and down in accordance with the loosened and tensed states of the sheet.

In the technique described in Japanese Unexamined Patent Application Publication No. 2003-306260, second conveying means is provided downstream of first conveying means in a sheet conveying direction. The second conveying means includes a plurality of driven rollers at a plurality of positions different from one another in the sheet conveying direction. The urging force applied by a driven roller of the first conveying means to a sheet is smaller than the urging force applied by the other driven rollers.

SUMMARY OF THE INVENTION

Meanwhile, a recording sheet may warp due to the manufacturing process or storage state of the recording sheet.

When the front edge of the warping sheet hits the circumferential surfaces of the pair of conveying rollers, the conveying speed of the recording sheet is temporarily decreased, and an image printed on the recording sheet may have defects such that lines appear and unevenness in printing frequently occurs.

Any of the techniques described above of the related art does not consider about warping of a sheet. Thus, the techniques of the related art are not sufficient for addressing the defects.

It is desirable to provide a printer that is advantageously assure the quality of a printed image even when a warping sheet is used.

A printer according to an embodiment of the present invention includes a driving roller conveyance mechanism and a driven roller conveyance mechanism both arranged downstream of a print section in a conveying direction of a sheet with a conveyance path interposed between the driving roller conveyance mechanism and the driven roller conveyance mechanism. The driving roller conveyance mechanism includes a first driving roller, a second driving roller, a first driving mechanism, and a second driving mechanism. The first driving roller has an outer circumferential surface made of a material having a large friction coefficient. The first driving roller has a shaft center extending in a direction orthogonal to the conveying direction. The second driving roller has an outer circumferential surface made of a material having a smaller friction coefficient than the outer circumferential surface of the first driving roller. The second driving roller is coaxial with the first driving roller. The first driving mechanism is configured to rotate the first driving roller so that the sheet is conveyed toward a downstream side in the conveying direction. The second driving mechanism is configured to rotate the second driving roller at a higher rotational speed than a rotational speed of the first driving roller so that the sheet is conveyed toward the downstream side in the conveying direction. The driven roller conveyance mechanism includes a rotatable first driven roller, a rotatable second driven roller, a first urging mechanism, and a second urging mechanism. The first driven roller is in contact with the first driving roller. The first driven roller has a shaft center extending in the direction orthogonal to the conveying direction. The rotatable second driven roller is in contact with the second driving roller. The second driven roller has a shaft center extending in the direction orthogonal to the conveying direction. The second driven roller has a larger outer diameter than the first driven roller. The first urging mechanism urges the first driven roller in a direction in which the first driven roller contacts the first driving roller. The second urging mechanism urges the second driven roller in a direction in which the second driven roller contacts the second driving roller. The second driven roller is formed of a spur roller having a toothed part on an outer circumferential part of the second driven roller.

In the embodiment, the second driven roller and the second driving roller can lead the front edge of the recording sheet to the position at which the first driven roller and the first driving roller pinch the recording sheet.

With this leading, the recording sheet receives substantially no shock in the conveying direction. Accordingly, a highly qualified printed result without lines or unevenness in printing can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a printer according to an embodiment;

FIG. 2 is a perspective view of the printer without a print head;

FIG. 3 is a perspective view of a primary portion of the printer;

FIG. 4 is a cross-sectional view of a driving roller conveyance mechanism;

FIG. 5 is an explanatory diagram illustrating the print head and a sheet ejection section;

FIG. 6 is an enlarged view of the sheet ejection section;

FIG. 7 illustrates rotation loci of spur rollers of first and second driven rollers;

FIG. 8 illustrates the state in which the front edge of a recording sheet is near the sheet ejection section;

FIG. 9 illustrates the state in which the front edge of the recording sheet is in contact with a spur;

FIG. 10 illustrates the state in which the front edge of the recording sheet is being led to a nip by the second driven roller;

FIG. 11 illustrates the state in which the front edge of the recording sheet is being pinched at the nip; and

FIG. 12 illustrates the state in which the front edge of the recording sheet has passed through the nip.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will be described below with reference to the attached drawings.

First, the basic structure of a printer 10 according to this embodiment will be described.

FIG. 1 is a cross-sectional view of the printer 10 according to this embodiment.

FIG. 2 is a perspective view of the printer 10 without a print head 20. FIG. 3 is a perspective view of a primary portion of the printer 10.

Referring to FIG. 1, the printer 10 of this embodiment is an inkjet printer. The printer 10 performs printing by discharging ink from a fixed print head 20 onto a recording sheet 2A that is fed from a recording sheet roll 2.

The printer 10 does not have to be an inkjet printer, and may be, for example, a sublimation printer or the like in related art. The printer 10 does not have to use a recording sheet roll, and may use previously cut recording sheets having a predetermined size.

Referring to FIG. 1, the printer 10 includes a case 12. The printer 10 also includes a recording sheet roll container 14, a sheet feed section 16, a sheet ejection section 18, a print section 19, a cutter device 24, and a motor 26 (FIG. 2), these components being arranged in the case 12.

In FIG. 1, a conveyance path 4 extends in a horizontal plane. The recording sheet 2A pulled from the recording sheet roll 2 is conveyed through the conveyance path 4.

Guide plates 4A and 4B are arranged upstream of the print head 20 in a conveying direction A of the recording sheet 2A. The guide plates 4A and 4B respectively guide the upper surface (surface to be printed) and the lower surface (back surface) of the recording sheet 2A.

The sheet feed section 16 conveys the recording sheet 2A pulled from the recording sheet roll 2 to the print section 19. The print section 19 performs printing on the recording sheet 2A. Then, the sheet ejection section 18 conveys the printed recording sheet 2A to the cutter device 24. The cutter device 24 cuts the recording sheet 2A into a sheet having a predetermined size. The cut recording sheet 2A is ejected through an ejection port 1202 that is provided in the case 12, and the recording sheet 2A is ejected onto a sheet ejection tray or the like disposed outside the case 12.

In FIG. 1, guide plates 4C and 4D are arranged upstream of the cutter device 24 in the conveying direction A. The guide plates 4C and 4D respectively guide the upper surface (surface to be printed) and the lower surface (back surface) of the recording sheet 2A.

The recording sheet roll container 14 contains and holds the recording sheet roll 2 so that the recording sheet roll 2 is rotatable, by using a recording sheet roll supporting device of related art.

The sheet feed section 16 feeds the recording sheet 2A to the print section 19. The recording sheet 2A is pulled from the recording sheet roll 2 contained in the recording sheet roll container 14.

Referring to FIG. 2, the sheet feed section 16 includes a sheet feed roller 32 and a pinch roller 34 arranged with the conveyance path 4 interposed therebetween. The sheet feed roller 32 and the pinch roller 34 convey the recording sheet 2A toward the downstream side in the conveying direction A along the conveyance path 4.

The sheet feed section 16 includes a drive shaft 30 linked to the motor 26 through a belt pulley mechanism 36. The drive shaft 30 extends in a direction orthogonal to the conveying direction A. The sheet feed roller 32 is attached to the drive shaft 30 so that the sheet feed roller 32 rotates with the drive shaft 30.

The pinch roller 34 is opposite to the sheet feed roller 32 with respect to the conveyance path 4. The pinch roller 34 includes a plurality of pinch rollers 34 provided at an interval in the direction orthogonal to the conveying direction A of the recording sheet 2A.

That is, the sheet feed roller 32 is disposed below the conveyance path 4. The pinch rollers 34 are disposed above the conveyance path 4.

The pinch rollers 34 are respectively supported by one ends of arms 38 to be rotatable around an axis parallel to the drive shaft 30.

An intermediate portion of each of the arms 38 is supported swingably around a spindle 40. An urging member 42, such as an extension spring, is stretched between the case 12 and another end of each arm 38. The urging member 42 urges the outer circumferential surface of the corresponding pinch roller 34 in a direction in which the outer circumferential surface of the pinch roller 34 contacts the outer circumferential surface of the sheet feed roller 32.

Thus, the power of the motor 26 rotates the drive shaft 30 and the sheet feed roller 32. The pinch rollers 34 are rotated by following the conveyance of the recording sheet 2A while the pinch rollers 34 and the sheet feed roller 32 pinch the recording sheet 2A.

Referring to FIG. 1, the print section 19 includes the print head 20 and a table 22 for printing.

The print head 20 is arranged downstream of the sheet feed section 16 in the conveying direction A. The print head 20 is disposed above the conveyance path 4.

The print head 20 includes ink discharge nozzles 20Y, 20M, 20C, and 20K that respectively discharge inks of yellow, magenta, cyan, and black on a surface to be printed of the recording sheet 2A.

The ink discharge nozzles 20Y, 20M, 20C, and 20K are arranged in order from the upstream side toward the downstream side in the conveying direction A of the recording sheet 2A.

The print head 20 is a line head. The ink discharge nozzles 20Y, 20M, 20C, and 20K linearly extend in the direction orthogonal to the conveying direction A of the recording sheet 2A. That is, the ink discharge nozzles 20Y, 20M, 20C, and 20K linearly extend over the entire length in the width direction of a recording sheet 2A having a maximum printable size. In other words, the ink discharge nozzles 20Y, 20M, 20C, and 20K linearly extend over the entire length in the width direction of the conveyance path 4.

Referring to FIG. 1, the table 22 for printing is opposite to the print head 20 with respect to the conveyance path 4. In other words, the table 22 for printing is arranged below the conveyance path 4.

Referring to FIGS. 2 and 3, the table 22 for printing includes an ink absorber 44 and a plurality of guide ribs 46.

The ink absorber 44 is a rectangular plate extending in the conveying direction A of the recording sheet 2A and in the width direction. The ink absorber 44 is made of a material that absorbs ink. An upper surface 4402 of the ink absorber 44 is a flat plane.

The ink absorber 44 absorbs ink dropped from the print head 20, ink discharged from the print head 20 when the recording sheet 2A is not provided below the print head 20, suspended ink mist, etc., to prevent the recording sheet 2A from being contaminated with ink.

The plurality of guide ribs 46 guide the back surface of the recording sheet 2A in the conveying direction A.

The plurality of guide ribs 46 protrude upward from the upper surface 4402 of the ink absorber 44 through a plurality of openings formed in the ink absorber 44. The tops of the guide ribs 46 are located in a common flat plane parallel to the horizontal plane.

The plurality of guide ribs 46 provide a gap between the back surface of the recording sheet 2A and the ink absorber 44. Thus, ink absorbed by the ink absorber 44 can be prevented from adhering on the back surface of the recording sheet 2A.

Referring to FIG. 1, the cutter device 24 cuts in the width direction the recording sheet 2A conveyed by the sheet ejection section 18 after the print section 19 performs printing.

The cutter device 24 may use a structure of related art, such as a roller-type cutter or a slide-type cutter.

Next, the sheet ejection section 18, which is a feature of the present invention, will be described below in detail.

FIG. 4 is a cross-sectional view of a driving roller conveyance mechanism 50. FIG. 5 is an explanatory diagram illustrating the print head 20 and the sheet ejection section 18. FIG. 6 is an enlarged view of the sheet ejection section 18.

Referring to FIGS. 2 and 3, the sheet ejection section 18 includes a driving roller conveyance mechanism 50 and a driven roller conveyance mechanism 52 both arranged downstream of the print section 19 in the conveying direction A of the recording sheet 2A with the conveyance path 4 interposed therebetween.

Referring to FIG. 3, the driving roller conveyance mechanism 50 is arranged below the conveyance path 4. The driven roller conveyance mechanism 52 is arranged above the conveyance path 4.

The driving roller conveyance mechanism 50 includes a first driving roller 54, a second driving roller 56, a first driving mechanism 58, and a second driving mechanism 60.

The outer circumferential surface of the first driving roller 54 is made of a material having a large friction coefficient. The first driving roller 54 has the shaft center extending in the direction orthogonal to the conveying direction A.

The outer circumferential surface of the first driving roller 54 is made of, for example, rubber.

The first driving mechanism 58 rotates the first driving roller 54 so that the recording sheet 2A is conveyed toward the downstream side in the conveying direction A.

Referring to FIGS. 3 and 4, the first driving mechanism 58 includes a first drive shaft 62 having the shaft center extending in the direction orthogonal to the conveying direction A. The first driving roller 54 is attached to the first drive shaft 62 so that the first driving roller 54 rotates with the first drive shaft 62.

Referring to FIG. 2, the first drive shaft 62 is linked to the drive shaft 30 through a belt pulley mechanism 64, and hence rotated by the motor 26.

In this embodiment, referring to FIGS. 3 and 4, the first driving roller 54 includes a plurality of first driving rollers 54 provided at an interval in the longitudinal direction of the first drive shaft 62.

The outer circumferential surface of the second driving roller 56 is made of a material having a smaller friction coefficient than the outer circumferential surfaces of the first driving rollers 54. The second driving roller 56 is coaxial with the first driving rollers 54.

The outer circumferential surface of the second driving roller 56 is made of, for example, synthetic resin such as polyacetal resin (POM).

In this embodiment, referring to FIGS. 3 and 4, the second driving roller 54 includes a plurality of second driving rollers 56 provided at a plurality of positions on the first drive shaft 62 not occupied by the first driving rollers 54. The second driving rollers 56 are rotatably supported by the first drive shaft 62.

To be more specific, three second driving rollers 56 are provided at the center and both ends in the width direction of the conveyance path 4. Two first driving rollers 54 are provided among the three second driving rollers 56, at both sides of the center of the conveyance path 4.

The outer diameter of the first driving rollers 54 and the outer diameter of the second driving rollers 56 may be equivalent to one another or may be different from one another. The equivalent diameters of the first and second driving rollers 54 and 56 are advantageous to smoothly convey the recording sheet 2A.

The second driving mechanism 60 rotates the second driving rollers 56 at a higher rotational speed than the rotational speed of the first driving rollers 54 so that the recording sheet 2A is conveyed toward the downstream side in the conveying direction A.

Referring to FIG. 3, the second driving mechanism 60 includes a second drive shaft 66, a first power transmission mechanism 68, and a second power transmission mechanism 70.

The second drive shaft 66 extends in parallel to the first drive shaft 62.

The first power transmission mechanism 68 links the first drive shaft 62 with the second drive shaft 66.

In this embodiment, the first power transmission mechanism 68 is formed of a belt pulley mechanism 6808 including a pulley 6802 attached to the first drive shaft 62, a pulley 6804 attached to the second drive shaft 66, and a belt 6806 hung over the pulleys 6802 and 6804.

The second power transmission mechanism 70 links the second drive shaft 66 with the corresponding second driving roller 56.

In this embodiment, referring to FIGS. 3 and 4, a small-diameter part 5602 is formed at an end in the longitudinal direction of each second driving roller 56. A pulley 7002 is attached to the second drive shaft 66 at a position corresponding to the small-diameter part 5602.

Also, a belt 7004 is hung over the small diameter part 5602 of the second driving roller 56 and the pulley 7002. Thus, the second power transmission mechanism 70 includes the small-diameter part 5602, the pulley 7002, and the belt 7004.

At least one of the first power transmission mechanism 68 and the second power transmission mechanism 70 rotates the second driving rollers 56 at a higher rotational speed than the rotational speed of the first drive shaft 62.

In this embodiment, the pulley 7002 has a larger outer diameter than the small-diameter part 5602. Thus, the second driving rollers 56 rotate at a higher rotational speed than the rotational speed of the first drive shaft 62.

Accordingly, a second driven roller 74 rotates at a higher rotational speed than the rotational speed of a first driven roller 72 (described below in detail). In this embodiment, when the second driven roller 74 directly contacts the corresponding second driving roller 56 and hence the second driven roller 74 is rotated, the rotational speed of the second driven roller 74 is about 1.2 to 1.5 times the rotational speed of the first driven roller 72.

Referring to FIGS. 3, 5, and 6, the driven roller conveyance mechanism 52 includes the first driven roller 72, the second driven roller 74, a first urging mechanism, and a second urging mechanism.

The first driven roller 72 is in contact with the first driving roller 54, has the shaft center in the direction orthogonal to the conveying direction A, and is rotatable.

In this embodiment, the first driven roller 72 is a spur roller having a toothed part on an outer circumferential part of the first driven roller 72.

The first driven roller 72 includes a plurality of first driven rollers 72 corresponding to the plurality of first driving rollers 54.

In this embodiment, two first driven rollers 72 are provided for every first driving roller 54.

Each second driven roller 74 is in contact with the corresponding second driving roller 56, has the shaft center extending in the direction orthogonal to the conveying direction A, has a larger outer diameter than the first driven roller 72, and is rotatable.

In this embodiment, the second driven roller 74 is a spur roller having a toothed part on an outer circumferential part of the second driven roller 74.

The second driven roller 74 includes a plurality of second driven rollers 74 corresponding to the plurality of second driving rollers 56.

In this embodiment, the second driven rollers 74 are respectively provided for the three second driving rollers 56 one by one.

The first urging mechanism urges each first driven roller 72 in a direction in which the first driven roller 72 contacts the corresponding first driving roller 54.

The second urging mechanism urges each second driven roller 74 in a direction in which the second driven roller 74 contacts the corresponding second driving roller 56.

The second driving roller 56 and the second driven roller 74 lead a front edge 2B of the recording sheet 2A being in contact with the second driven roller 74 to a first nip 80, which will be described below.

The urging force for urging the second driven roller 74 to the second driving roller 56 is smaller than the urging force for urging the first driven roller 72 to the first driving roller 54. Thus, the conveyance of the recording sheet 2A by the first driving roller 54 and the first driven roller 72 is stabilized.

The first and second urging members may use any of various structures of related art, such as the arms 38, the spindle 40, and the urging members 42, which have been described above in the description for the pinch rollers 34.

Now, the operation of the printer 10 will be described.

Referring to FIGS. 1 and 5, assume that the recording sheet 2A pulled from the recording sheet roll container 14 is pinched between the sheet feed roller 32 and the pinch rollers 34 in advance.

In this state, when the motor 26 rotates the sheet feed roller 32 with the drive shaft 30, the sheet feed section 16 conveys the recording sheet 2A to the print section 19 at a constant conveying speed.

When the recording sheet 2A reaches the print section 19, the ink discharge nozzles 20Y, 20M, 20C, and 20K eject inks on the surface to be printed of the recording sheet 2A and thus an image is printed on the surface of the recording sheet 2A.

Referring to FIG. 5, when the front edge 2B of the recording sheet 2A reaches the sheet ejection section 18, the recording sheet 2A is conveyed by the sheet feed section 16 and the sheet ejection section 18 continuously at the constant conveying speed, and in this conveying state, the print section 19 performs printing.

Referring to FIG. 1, the sheet ejection section 18 ejects the recording sheet 2A after printing through the ejection port 1202. The cutter device 24 cuts the recording sheet 2A into a predetermined size. The cut recording sheet 2A is ejected onto a sheet ejection tray.

Next, the operation of the sheet ejection section 18 will be described below in detail.

FIG. 7 illustrates rotation loci of the spur rollers of the first and second driven rollers 72 and 74. FIG. 8 illustrates the state in which the front edge 2B of the recording sheet 2A is near the sheet ejecting section 18. FIG. 9 illustrates the state in which the front edge 2B of the recording sheet 2A is in contact with a spur. FIG. 10 illustrates the state in which the front edge 2B of the recording sheet 2A is being led to the nip 80 by the second driven roller 74. FIG. 11 illustrates the state in which the front edge 2B of the recording sheet 2A is being pinched at the nip 80. FIG. 12 illustrates the state in which the front edge 2B of the recording sheet 2A has passed through the nip 80.

In FIGS. 7 to 11, reference numeral 80 denotes a first nip at which the first driven roller 72 and the first driving roller 54 pinch the recording sheet 2A, and reference numeral 82 denotes a second nip at which the second driven roller 74 and the second driving roller 56 pinch the recording sheet 2A.

Referring to FIGS. 5, 6, and 8, a case will be described in which the recording sheet 2A with the entire front edge 2B in the width direction warping upward enters the sheet ejection section 18.

Referring to FIG. 9, the front edge 2B of the recording sheet 2A contacts the second driven roller 74.

The second driven roller 74 has a larger outer diameter than the first driven roller 72, as indicated by the rotation locus of the addendum (tip part) of the gear of each spur roller defining the second driven roller 74 in FIG. 7.

Thus, the front edge 2B of the recording sheet 2A lifted up from the first nip 80 contacts the second driven roller 74. Hence, the front edge 2B does not directly hit the first driven roller 72.

In addition, the rotational speed of the second driven roller 74 is higher than the rotational speed of the first driven roller 72. Thus, the circumferential speed of the second driven roller 74 is higher than the circumferential speed of the first driven roller 72.

The second driven roller 74 gently flips the front edge 2B of the recording sheet 2A which has contacted the second driven roller 74, and leads the front edge 2B quickly to the first nip 80.

The recording sheet 2A receives substantially no shock. The front edge 2B is smoothly led to the first nip 80.

As the second driven roller 74 has a larger diameter, a direction of a force acting on the front edge 2B when the front edge 2B has contacted the second driven roller 74 can be closer to the conveying direction A of the recording sheet 2A. This arrangement is advantageous to smoothly lead the front edge 2B.

In this embodiment, the urging force for urging the second driven roller 74 to the second driving roller 56 is smaller than the urging force for urging the first driven roller 72 to the first driving roller 54.

Hence, the second driven roller 74 has a small moment of inertia. Thus, the force applied to the front edge 2B of the recording sheet 2A which has contacted the second driven roller 74 becomes smaller than the force applied by the first driven roller 72 to cause the recording sheet 2A to be conveyed in the conveying direction A. Therefore, this arrangement is advantageous to stabilize the conveyance of the recording sheet 2A by the first driving roller 54 and the first driven roller 72.

Then, referring to FIG. 10, the front edge 2B of the recording sheet 2A reaches the first nip 80 because of leading with the second driven roller 74.

In this embodiment, the first driving roller 54 has the outer diameter equivalent to the outer diameter of the second driving roller 56.

The first nip 80 and the second nip 82 are located along a line in the width direction of the recording sheet 2A.

Thus, theoretically, the front edge 2B of the recording sheet 2A does not contact the teeth of the first driven roller 72 until the front edge 2B reaches the first nip 80 by leading with the second driven roller 74.

Referring to FIG. 11, the recording sheet 2A attains a designed sheet passage state at the first nip 80. The first driving roller 54 and the first driven roller 72 bite the front edge 2B of the recording sheet 2A.

The recording sheet 2A is pinched by the first driving roller 54 and the first driven roller 72 with a sufficient nip pressure and further conveyed toward the downstream side.

In particular, the recording sheet 2A is pinched by the first driving roller 54 and the first driven roller 72 at the first nip 80, and at the same time, the recording sheet 2A is pinched by the second driving roller 56 and the second driven roller 74 at the second nip 82.

The outer circumferential surface of the second driving roller 56 is made of a material having a smaller friction coefficient than the outer circumferential surface of the first driving roller 54. Thus, the second driving roller 56 is idly rotated even when the second driving roller 56 contacts the recording sheet 2A. The second driven roller 74 is rotated by following the conveyance of the recording sheet 2A.

Hence, the conveying force applied by the second driving roller 56 and the second driven roller 74 to the recording sheet 2A is substantially negligible. Therefore, this arrangement is advantageous to maintain the conveying speed to be constant without an adverse effect for the conveying operation of the recording sheet 2A applied by the first driving roller 54 and the first driven roller 72.

With this embodiment, the second driven roller 74 and the second driving roller 56 lead the front edge 2B of the recording sheet 2A to the first nip 80 at which the first driven roller 72 and the first driving roller 54 pinch the recording sheet 2A. Thus, the recording sheet 2A receives substantially no shock in the conveying direction A.

The conveying speed of the recording sheet 2A can be prevented from being temporarily decreased. The unstable conveyance can be prevented during printing. Accordingly, this embodiment is advantageous to obtain a highly qualified printed result without lines or unevenness in printing.

Also, the recording sheet 2A can be prevented from being stacked. Malfunction of the printer 10 can be prevented, and the recording sheet 2A is not wasted. This embodiment is advantageous to increase the commercial value of the printer 10.

Further, in this embodiment, since the first driven roller 72 and the second driven roller 74 are formed of the spur rollers having the toothed parts on the outer circumferential parts thereof, ink does not adhere onto the recording sheet 2A although the first driven roller 72 and the second driven roller 74 contact the surface to be printed of the recording sheet 2A.

Thus, the situation, in which the ink adhering on the first driven roller 72 and the second driven roller 74 is transferred onto the surface to be printed of the recording sheet 2A and the surface to be printed is contaminated because of the transferred ink, can be prevented. In particular, this embodiment is advantageous when the print head 20 is an inkjet print head.

The present application contains subject matter related to that disclosed in Japanese Priority Patent Application JP 2008-310503 filed in the Japan Patent Office on Dec. 5, 2008, the entire content of which is hereby incorporated by reference.

It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof. 

1. A printer comprising: a driving roller conveyance mechanism and a driven roller conveyance mechanism both arranged downstream of a print section in a conveying direction of a sheet with a conveyance path interposed between the driving roller conveyance mechanism and the driven roller conveyance mechanism, wherein the driving roller conveyance mechanism includes a first driving roller having an outer circumferential surface made of a material having a large friction coefficient, the first driving roller having a shaft center extending in a direction orthogonal to the conveying direction, a second driving roller having an outer circumferential surface made of a material having a smaller friction coefficient than the outer circumferential surface of the first driving roller, the second driving roller being coaxial with the first driving roller, a first driving mechanism configured to rotate the first driving roller so that the sheet is conveyed toward a downstream side in the conveying direction, and a second driving mechanism configured to rotate the second driving roller at a higher rotational speed than a rotational speed of the first driving roller so that the sheet is conveyed toward the downstream side in the conveying direction, wherein the driven roller conveyance mechanism includes a rotatable first driven roller being in contact with the first driving roller, the first driven roller having a shaft center extending in the direction orthogonal to the conveying direction, a rotatable second driven roller being in contact with the second driving roller, the second driven roller having a shaft center extending in the direction orthogonal to the conveying direction, the second driven roller having a larger outer diameter than the first driven roller, a first urging mechanism urging the first driven roller in a direction in which the first driven roller contacts the first driving roller, and a second urging mechanism urging the second driven roller in a direction in which the second driven roller contacts the second driving roller, and wherein the second driven roller is formed of a spur roller having a toothed part on an outer circumferential part of the second driven roller.
 2. The printer according to claim 1, wherein the first driving mechanism includes a first drive shaft having a shaft center extending in the direction orthogonal to the conveying direction, wherein the first driving roller is attached to the first drive shaft so that the first driving roller rotates with the first drive shaft, wherein the second driving roller is rotatably supported by the first drive shaft, wherein the second driving mechanism includes a second drive shaft extending in parallel to the first drive shaft, a first power transmission mechanism linking the first drive shaft with the second drive shaft, and a second power transmission mechanism linking the second drive shaft with the second driving roller, and wherein at least one of the first power transmission mechanism and the second power transmission mechanism rotates the second driving roller at a higher rotational speed than a rotational speed of the first drive shaft.
 3. The printer according to claim 2, wherein the second driving roller includes a plurality of second driving rollers provided at an interval in a longitudinal direction of the first drive shaft, and wherein the second driven roller includes a plurality of second driven rollers corresponding to the plurality of second driving rollers.
 4. The printer according to claim 2, wherein the first driving roller includes a plurality of first driving rollers provided at an interval in a longitudinal direction of the first drive shaft, wherein the second driving roller includes a plurality of second driving rollers provided at a plurality of positions on the first drive shaft not occupied by the first driving rollers, the second driving rollers rotatably supported by the first drive shaft, wherein the first driven roller includes a plurality of first driven rollers corresponding to the plurality of first driving rollers, and wherein the second driven roller includes a plurality of second driven rollers corresponding to the plurality of second driving rollers.
 5. The printer according to claim 2, wherein the conveyance path has a width orthogonal to the conveying direction, wherein the second driving roller includes three second driving rollers disposed at the center and both ends in a width direction of the conveyance path, wherein the first driving roller includes two first driving rollers disposed among the three second driving rollers, at both sides of the center of the conveyance path, wherein the first driven roller includes a plurality of first driven rollers corresponding to the plurality of first driving rollers, and wherein the second driven roller includes a plurality of second driven rollers corresponding to the plurality of second driving rollers.
 6. The printer according to any of claims 1 to 5, wherein the second urging mechanism provides a smaller urging force for urging the second driven roller in the direction in which the second driven roller contacts the second driving roller than an urging force of the first urging mechanism for urging the first driven roller in the direction in which the first driven roller contacts the first driving roller.
 7. The printer according to any of claims 1-5, wherein the outer circumferential surface of the first driving roller is made of rubber, and wherein the outer circumferential surface of the second driving roller is made of synthetic resin having a smaller friction coefficient than the rubber.
 8. The printer according to any of claims 1-5, wherein the first driven roller is formed of a spur roller having a toothed part on an outer circumferential part of the first driven roller.
 9. The printer according to any of claims 1-5, wherein the first driving roller has an outer diameter equivalent to the second driving roller. 